To meet the recent demand for higher integration in integrated circuits, pattern formation to a finer feature size is required. Acid-catalyzed chemically amplified resist compositions are most often used in forming resist patterns with a feature size of 0.2 μm or less. High-energy radiation such as UV, deep UV or electron beam (EB) is used as the light source for exposure of these resist compositions. In particular, while EB lithography is utilized as the ultra-fine microfabrication technique, it is also indispensable in processing a photomask blank to form a photomask for use in semiconductor device fabrication.
In general, the EB lithography is by writing an image with EB, without using a mask. In the case of positive resist, those regions of a resist film other than the regions to be retained are successively irradiated with EB having a minute area. The operation of successively scanning all finely divided regions on the work surface takes a long time as compared with full wafer exposure through a photomask. In order to avoid any decline of throughput, the resist film must be highly sensitive. Because of the long image-writing time, there is a likelihood of a difference arising between the initially written portion and the later written portion. Thus the stability with time of exposed regions in vacuum is one of important performance requirements. One of the important applications of chemically amplified resist material resides in processing of mask blanks. Some mask blanks have a surface material that can have an impact on the pattern profile of the overlying chemically amplified resist film, such as a layer of a chromium compound, typically chromium oxide deposited on a photomask substrate. For high resolution and profile retention after etching, it is one important performance factor to maintain the pattern profile of resist film rectangular independent of the type of substrate.
The control of resist sensitivity and pattern profile as mentioned above has been improved by a proper selection and combination of resist material-constituting components and processing conditions. One outstanding problem is the diffusion of acid that largely affects the resolution of a chemically amplified resist film. In processing of photomasks, it is required that the profile of a resist pattern formed as above do not change with a lapse of time from the end of exposure to post-exposure baking. The major cause of such a change with time is diffusion of an acid generated upon exposure. The problem of acid diffusion has been widely studied not only in the field of photomask processing, but also in the field of general resist films because it has a significant impact on sensitivity and resolution.
In particular, JP-A H09-325497 discloses that a sulfonic acid to be generated upon exposure is incorporated into a resin for use in resist material for inhibiting acid diffusion. This control method is attractive since it relies on a mechanism different from the control method using a base. Various improvements have been made on this method to comply with the demand of forming finer size patterns. JP-A 2008-133448 is a useful example of achieving an improvement in acid strength.
Another important material for inhibiting acid diffusion is a basic compound. Many modifications of the basic compound have been reported. For example, in conjunction with the resist composition for use in the ArF immersion lithography involving exposure of a resist film to ArF excimer laser light in a setup that a water layer is formed on the resist film, JP-A 2008-133312 proposes to use a polymer having a basic compound bound thereto in order to prevent the basic compound from migrating and diffusing into the water phase in contact with the resist film to alter the resolution of the resist surface region.
As an exemplary polymer endowed with a nitrogen-containing partial structure, a resist composition using a heterocycle-bearing polymer is disclosed in JP-A 2009-86310 although this is not for the purpose of restraining acid diffusion.